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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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READ(3P) POSIX Programmer's Manual READ(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
pread, read — read from a file
#include <unistd.h>
ssize_t pread(int fildes, void *buf, size_t nbyte, off_t offset);
ssize_t read(int fildes, void *buf, size_t nbyte);
The read() function shall attempt to read nbyte bytes from the
file associated with the open file descriptor, fildes, into the
buffer pointed to by buf. The behavior of multiple concurrent
reads on the same pipe, FIFO, or terminal device is unspecified.
Before any action described below is taken, and if nbyte is zero,
the read() function may detect and return errors as described
below. In the absence of errors, or if error detection is not
performed, the read() function shall return zero and have no other
results.
On files that support seeking (for example, a regular file), the
read() shall start at a position in the file given by the file
offset associated with fildes. The file offset shall be
incremented by the number of bytes actually read.
Files that do not support seeking—for example, terminals—always
read from the current position. The value of a file offset
associated with such a file is undefined.
No data transfer shall occur past the current end-of-file. If the
starting position is at or after the end-of-file, 0 shall be
returned. If the file refers to a device special file, the result
of subsequent read() requests is implementation-defined.
If the value of nbyte is greater than {SSIZE_MAX}, the result is
implementation-defined.
When attempting to read from an empty pipe or FIFO:
* If no process has the pipe open for writing, read() shall
return 0 to indicate end-of-file.
* If some process has the pipe open for writing and O_NONBLOCK
is set, read() shall return -1 and set errno to [EAGAIN].
* If some process has the pipe open for writing and O_NONBLOCK
is clear, read() shall block the calling thread until some
data is written or the pipe is closed by all processes that
had the pipe open for writing.
When attempting to read a file (other than a pipe or FIFO) that
supports non-blocking reads and has no data currently available:
* If O_NONBLOCK is set, read() shall return -1 and set errno to
[EAGAIN].
* If O_NONBLOCK is clear, read() shall block the calling thread
until some data becomes available.
* The use of the O_NONBLOCK flag has no effect if there is some
data available.
The read() function reads data previously written to a file. If
any portion of a regular file prior to the end-of-file has not
been written, read() shall return bytes with value 0. For example,
lseek() allows the file offset to be set beyond the end of
existing data in the file. If data is later written at this point,
subsequent reads in the gap between the previous end of data and
the newly written data shall return bytes with value 0 until data
is written into the gap.
Upon successful completion, where nbyte is greater than 0, read()
shall mark for update the last data access timestamp of the file,
and shall return the number of bytes read. This number shall
never be greater than nbyte. The value returned may be less than
nbyte if the number of bytes left in the file is less than nbyte,
if the read() request was interrupted by a signal, or if the file
is a pipe or FIFO or special file and has fewer than nbyte bytes
immediately available for reading. For example, a read() from a
file associated with a terminal may return one typed line of data.
If a read() is interrupted by a signal before it reads any data,
it shall return -1 with errno set to [EINTR].
If a read() is interrupted by a signal after it has successfully
read some data, it shall return the number of bytes read.
For regular files, no data transfer shall occur past the offset
maximum established in the open file description associated with
fildes.
If fildes refers to a socket, read() shall be equivalent to recv()
with no flags set.
If the O_DSYNC and O_RSYNC bits have been set, read I/O operations
on the file descriptor shall complete as defined by synchronized
I/O data integrity completion. If the O_SYNC and O_RSYNC bits have
been set, read I/O operations on the file descriptor shall
complete as defined by synchronized I/O file integrity completion.
If fildes refers to a shared memory object, the result of the
read() function is unspecified.
If fildes refers to a typed memory object, the result of the
read() function is unspecified.
A read() from a STREAMS file can read data in three different
modes: byte-stream mode, message-nondiscard mode, and message-
discard mode. The default shall be byte-stream mode. This can be
changed using the I_SRDOPT ioctl() request, and can be tested with
I_GRDOPT ioctl(). In byte-stream mode, read() shall retrieve data
from the STREAM until as many bytes as were requested are
transferred, or until there is no more data to be retrieved.
Byte-stream mode ignores message boundaries.
In STREAMS message-nondiscard mode, read() shall retrieve data
until as many bytes as were requested are transferred, or until a
message boundary is reached. If read() does not retrieve all the
data in a message, the remaining data shall be left on the STREAM,
and can be retrieved by the next read() call. Message-discard mode
also retrieves data until as many bytes as were requested are
transferred, or a message boundary is reached. However, unread
data remaining in a message after the read() returns shall be
discarded, and shall not be available for a subsequent read(),
getmsg(), or getpmsg() call.
How read() handles zero-byte STREAMS messages is determined by the
current read mode setting. In byte-stream mode, read() shall
accept data until it has read nbyte bytes, or until there is no
more data to read, or until a zero-byte message block is
encountered. The read() function shall then return the number of
bytes read, and place the zero-byte message back on the STREAM to
be retrieved by the next read(), getmsg(), or getpmsg(). In
message-nondiscard mode or message-discard mode, a zero-byte
message shall return 0 and the message shall be removed from the
STREAM. When a zero-byte message is read as the first message on a
STREAM, the message shall be removed from the STREAM and 0 shall
be returned, regardless of the read mode.
A read() from a STREAMS file shall return the data in the message
at the front of the STREAM head read queue, regardless of the
priority band of the message.
By default, STREAMs are in control-normal mode, in which a read()
from a STREAMS file can only process messages that contain a data
part but do not contain a control part. The read() shall fail if a
message containing a control part is encountered at the STREAM
head. This default action can be changed by placing the STREAM in
either control-data mode or control-discard mode with the I_SRDOPT
ioctl() command. In control-data mode, read() shall convert any
control part to data and pass it to the application before passing
any data part originally present in the same message. In control-
discard mode, read() shall discard message control parts but
return to the process any data part in the message.
In addition, read() shall fail if the STREAM head had processed an
asynchronous error before the call. In this case, the value of
errno shall not reflect the result of read(), but reflect the
prior error. If a hangup occurs on the STREAM being read, read()
shall continue to operate normally until the STREAM head read
queue is empty. Thereafter, it shall return 0.
The pread() function shall be equivalent to read(), except that it
shall read from a given position in the file without changing the
file offset. The first three arguments to pread() are the same as
read() with the addition of a fourth argument offset for the
desired position inside the file. An attempt to perform a pread()
on a file that is incapable of seeking shall result in an error.
Upon successful completion, these functions shall return a non-
negative integer indicating the number of bytes actually read.
Otherwise, the functions shall return -1 and set errno to indicate
the error.
These functions shall fail if:
EAGAIN The file is neither a pipe, nor a FIFO, nor a socket, the
O_NONBLOCK flag is set for the file descriptor, and the
thread would be delayed in the read operation.
EBADF The fildes argument is not a valid file descriptor open for
reading.
EBADMSG
The file is a STREAM file that is set to control-normal
mode and the message waiting to be read includes a control
part.
EINTR The read operation was terminated due to the receipt of a
signal, and no data was transferred.
EINVAL The STREAM or multiplexer referenced by fildes is linked
(directly or indirectly) downstream from a multiplexer.
EIO The process is a member of a background process group
attempting to read from its controlling terminal, and
either the calling thread is blocking SIGTTIN or the
process is ignoring SIGTTIN or the process group of the
process is orphaned. This error may also be generated for
implementation-defined reasons.
EISDIR The fildes argument refers to a directory and the
implementation does not allow the directory to be read
using read() or pread(). The readdir() function should be
used instead.
EOVERFLOW
The file is a regular file, nbyte is greater than 0, the
starting position is before the end-of-file, and the
starting position is greater than or equal to the offset
maximum established in the open file description associated
with fildes.
The pread() function shall fail if:
EINVAL The file is a regular file or block special file, and the
offset argument is negative. The file offset shall remain
unchanged.
ESPIPE The file is incapable of seeking.
The read() function shall fail if:
EAGAIN The file is a pipe or FIFO, the O_NONBLOCK flag is set for
the file descriptor, and the thread would be delayed in the
read operation.
EAGAIN or EWOULDBLOCK
The file is a socket, the O_NONBLOCK flag is set for the
file descriptor, and the thread would be delayed in the
read operation.
ECONNRESET
A read was attempted on a socket and the connection was
forcibly closed by its peer.
ENOTCONN
A read was attempted on a socket that is not connected.
ETIMEDOUT
A read was attempted on a socket and a transmission timeout
occurred.
These functions may fail if:
EIO A physical I/O error has occurred.
ENOBUFS
Insufficient resources were available in the system to
perform the operation.
ENOMEM Insufficient memory was available to fulfill the request.
ENXIO A request was made of a nonexistent device, or the request
was outside the capabilities of the device.
The following sections are informative.
Reading Data into a Buffer
The following example reads data from the file associated with the
file descriptor fd into the buffer pointed to by buf.
#include <sys/types.h>
#include <unistd.h>
...
char buf[20];
size_t nbytes;
ssize_t bytes_read;
int fd;
...
nbytes = sizeof(buf);
bytes_read = read(fd, buf, nbytes);
...
None.
This volume of POSIX.1‐2017 does not specify the value of the file
offset after an error is returned; there are too many cases. For
programming errors, such as [EBADF], the concept is meaningless
since no file is involved. For errors that are detected
immediately, such as [EAGAIN], clearly the offset should not
change. After an interrupt or hardware error, however, an updated
value would be very useful and is the behavior of many
implementations.
Note that a read() of zero bytes does not modify the last data
access timestamp. A read() that requests more than zero bytes, but
returns zero, is required to modify the last data access
timestamp.
Implementations are allowed, but not required, to perform error
checking for read() requests of zero bytes.
Input and Output
The use of I/O with large byte counts has always presented
problems. Ideas such as lread() and lwrite() (using and returning
longs) were considered at one time. The current solution is to use
abstract types on the ISO C standard function to read() and
write(). The abstract types can be declared so that existing
functions work, but can also be declared so that larger types can
be represented in future implementations. It is presumed that
whatever constraints limit the maximum range of size_t also limit
portable I/O requests to the same range. This volume of
POSIX.1‐2017 also limits the range further by requiring that the
byte count be limited so that a signed return value remains
meaningful. Since the return type is also a (signed) abstract
type, the byte count can be defined by the implementation to be
larger than an int can hold.
The standard developers considered adding atomicity requirements
to a pipe or FIFO, but recognized that due to the nature of pipes
and FIFOs there could be no guarantee of atomicity of reads of
{PIPE_BUF} or any other size that would be an aid to applications
portability.
This volume of POSIX.1‐2017 requires that no action be taken for
read() or write() when nbyte is zero. This is not intended to take
precedence over detection of errors (such as invalid buffer
pointers or file descriptors). This is consistent with the rest of
this volume of POSIX.1‐2017, but the phrasing here could be
misread to require detection of the zero case before any other
errors. A value of zero is to be considered a correct value, for
which the semantics are a no-op.
I/O is intended to be atomic to ordinary files and pipes and
FIFOs. Atomic means that all the bytes from a single operation
that started out together end up together, without interleaving
from other I/O operations. It is a known attribute of terminals
that this is not honored, and terminals are explicitly (and
implicitly permanently) excepted, making the behavior unspecified.
The behavior for other device types is also left unspecified, but
the wording is intended to imply that future standards might
choose to specify atomicity (or not).
There were recommendations to add format parameters to read() and
write() in order to handle networked transfers among heterogeneous
file system and base hardware types. Such a facility may be
required for support by the OSI presentation of layer services.
However, it was determined that this should correspond with
similar C-language facilities, and that is beyond the scope of
this volume of POSIX.1‐2017. The concept was suggested to the
developers of the ISO C standard for their consideration as a
possible area for future work.
In 4.3 BSD, a read() or write() that is interrupted by a signal
before transferring any data does not by default return an [EINTR]
error, but is restarted. In 4.2 BSD, 4.3 BSD, and the Eighth
Edition, there is an additional function, select(), whose purpose
is to pause until specified activity (data to read, space to
write, and so on) is detected on specified file descriptors. It is
common in applications written for those systems for select() to
be used before read() in situations (such as keyboard input) where
interruption of I/O due to a signal is desired.
The issue of which files or file types are interruptible is
considered an implementation design issue. This is often affected
primarily by hardware and reliability issues.
There are no references to actions taken following an
``unrecoverable error''. It is considered beyond the scope of this
volume of POSIX.1‐2017 to describe what happens in the case of
hardware errors.
Earlier versions of this standard allowed two very different
behaviors with regard to the handling of interrupts. In order to
minimize the resulting confusion, it was decided that POSIX.1‐2008
should support only one of these behaviors. Historical practice on
AT&T-derived systems was to have read() and write() return -1 and
set errno to [EINTR] when interrupted after some, but not all, of
the data requested had been transferred. However, the US
Department of Commerce FIPS 151‐1 and FIPS 151‐2 require the
historical BSD behavior, in which read() and write() return the
number of bytes actually transferred before the interrupt. If -1
is returned when any data is transferred, it is difficult to
recover from the error on a seekable device and impossible on a
non-seekable device. Most new implementations support this
behavior. The behavior required by POSIX.1‐2008 is to return the
number of bytes transferred.
POSIX.1‐2008 does not specify when an implementation that buffers
read()s actually moves the data into the user-supplied buffer, so
an implementation may choose to do this at the latest possible
moment. Therefore, an interrupt arriving earlier may not cause
read() to return a partial byte count, but rather to return -1 and
set errno to [EINTR].
Consideration was also given to combining the two previous
options, and setting errno to [EINTR] while returning a short
count. However, not only is there no existing practice that
implements this, it is also contradictory to the idea that when
errno is set, the function responsible shall return -1.
This volume of POSIX.1‐2017 intentionally does not specify any
pread() errors related to pipes, FIFOs, and sockets other than
[ESPIPE].
None.
fcntl(3p), ioctl(3p), lseek(3p), open(3p), pipe(3p), readv(3p)
The Base Definitions volume of POSIX.1‐2017, Chapter 11, General
Terminal Interface, stropts.h(0p), sys_uio.h(0p), unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 READ(3P)
Pages that refer to this page: aio.h(0p), stropts.h(0p), sys_uio.h(0p), unistd.h(0p), aio_error(3p), aio_fsync(3p), aio_read(3p), aio_return(3p), fchmod(3p), fdatasync(3p), fstatvfs(3p), futimens(3p), getmsg(3p), ioctl(3p), lio_listio(3p), lockf(3p), open(3p), pipe(3p), poll(3p), pread(3p), pselect(3p), putmsg(3p), readv(3p), recv(3p), recvfrom(3p), shutdown(3p), stdin(3p), ungetc(3p), ungetwc(3p), write(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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